It is the understanding of those skilled in the art that if the content of carbon (C), phosphorus (P) and nitrogen (N) of stainless steels is much more reduced than performed today, very excellent materials will be obtained. That is, it is well known that good toughness and corrosion resistance are achieved by reducing the content of C and N, and hot cracking and stress corrosion cracking can be avoided by reducing the content of P and N. Although now the C content can be reduced considerably, there is no effective and economical method for reducing the content of P and N of the stainless steels.
The reason why dephosphorization and denitrification of stainless steels is difficult is that Cr increases solubility of N in iron, and Cr is oxidized preferentially to P. The solubility of N in molten pig iron containing 3% Cr is about 1.5 times as much as that in plain pig iron. As to P, very low P content stainlsss steels can be produced by carefully selecting low P content materials, although the resulting products inevitably become of high price. But as to N, we cannot resort to such means. Therefore, for the purpose of denitrifying the Cr pig iron, not a few complicated methods have been proposed, such as combination of vacuum melting and electron-beam melting, or denitrifying the Cr pig iron with aluminum (Al) first and thereafter oxidizing the residual Al together with C. However, the combination of evacuation melting and electron-beam melting requires costly equipment and operation cost is high, too. The denitrification by means of Al as disclosed in Japanese Laying-Open Patent Publication No. 98318/74, for instance, requires removal of the Al that remains in iron in an amount of order of several percents. Further the formed Al nitride must be separated from molten iron. But some portion of AlN remains in the molten iron, which decomposes at the later decarburization stage and the N dissolves in the iron again.
As for the dephosphorization of plain pig iron, rather recently, it has been proposed for that purpose to incorporate oxides, carbonates, chlorides of alkali metals in the smelting slags. For instance, in Japanese Laying-Open Patent Publication No. 2322/78, "a dephosphorization agent to be used for dephosphorizing molten pig iron comprising a mixture of lime, iron ore, soda ash and fluorite, characterized in that iron oxide is added in an amount not less than 2.5 times the weight of the oxide or carbonate of an alkali metal, the ingredients are mixed and pulverized and heated at 600.degree. C. or higher so that compounds of iron oxides and alkali metal oxides are formed, and CaO is added in an amount from equal with to 10 times the amount of said compounds" is disclosed. In Japanese Laying-Open Patent Publication No. 26715/78, "an auxiliary refining agent for molten iron containing an alkali metal compound, to which a SiO.sub.2 -containing material containing not less than 50% SiO.sub.2 and/or a SiO.sub.2 -containing material in which the total content of SiO.sub.2, Na.sub.2 O, MnO and FeO is not less than 60% is added, whereby the amount of SiO.sub.2 and the SiO.sub.2 -containing material is respectively 20% or less and 50% or less" is disclosed. Further in Japanese Laying-Open Patent Publication No. 28511/78, "a dephosphorization, desulfurization or dephosphorization-desulfurization slag comprising 30-70% CaO, 10-40% CaF.sub.2 as the principal ingredients, and 1-30% of at least one of Na.sub.2 O, B.sub.2 O.sub.5, Na.sub.2 B.sub.4 O.sub.7, K.sub.2 O, Li.sub.2 O, NaCl, KCl, and LiCl" is disclosed.
However, all these slags or refining agents may be effective for plain pig iron, but they are quite ineffective for dephosphorization of the Cr pig iron. All the descriptions of these three quoted Japanese Laying-Open Patent Publications relate to dephosphorization of plain pig iron and there is no reference to dephosphorization of the Cr pig iron.
Difficulty of dephosphorization of the Cr pig iron is considered to be as follows.
The oxidation reactions of P, Cr and iron (Fe) are regarded to be as follows:
______________________________________ ##STR1## 3.73 .times. 10.sup.-9 atm (1) ##STR2## 5.07 .times. 10.sup.-14 atm (2) Fe + 1/2 O.sub.2 = FeO 1.37 .times. 10.sup.-9 atm (3) ______________________________________
The numerical value for pressure indicated on the right side of each equation represents the equilibrium oxygen (O) partial pressure under the standard state at 1500.degree. C. for each substance. It will be learned from these data that Cr combines with oxygen far easier than P and Fe. This fact is one of the reasons that the dephosphorization of the molten Cr pig iron is extremely difficult in comparison with that of molten plain pig iron containing no Cr. That is to say, in the prior art processes, the intention to dephosphorize by oxidation resulted in oxidation of Cr only, and oxidation of P did not occur. Even if P is oxidized, Cr is oxidized far more. Also it has been learned that the produced oxide of Cr (referred to as Cr-.sub.2 O.sub.3) impairs dephosphorizing power of the slag. It is understood that the formed Cr.sub.2 O.sub.3 acts as an acidic oxide and combines with P.sub.2 O.sub.5 -fixing materials and substantially reduces their P.sub.2 O.sub.5 -fixing ability. That is, in the case of the Cr pig iron, the fixation of the formed P.sub.2 O.sub.5 is difficult, that is, the so-called rephosphorization becomes a serious problem.
Therefore, in order to carry out dephosphorization of the Cr pig iron, it is necessary to promote the reaction EQU 2P+5FeO.fwdarw.P.sub.2 O.sub.5 +5Fe (4)
and at the same time to control as much as possible the reaction EQU 2Cr+3FeO.fwdarw.Cr.sub.2 O.sub.3 +3Fe (5)
The known measures for oxidizing a molten iron bath as controlling oxidation of Cr therein are to reduce the partial pressure of CO of the atmosphere. Specifically speaking, it is known to reduce the pressure of the surrounding atmosphere or to contact a gaseous mixture of an oxidizing gas such as oxygen (O) and an inert gas such as argon (Ar) with the molten iron bath.
It is another means for dephosphorizing while controlling oxidation of Cr to reduce the oxygen potential of the iron bath. The decrease in the oxygen potential of the iron bath can be achieved by increase in the silicon (Si) content in the bath. But it is not desirable because Si is oxidized to SiO.sub.2, which lowers basicity of the slag. In this respect, carbon (C) is oxidized to produce CO which has no influence on the slag property. Therefore increase in the carbon content of the bath is preferred.
According to the knowledge hitherto, as noted in Japanese Laying-Open Patent Publication No. 28511/78 quoted above, which relates to the plain carbon steel, and foreseen from the above equation (1), it is thought that in order to promote oxidation of P, the oxygen potential of the iron bath should be raised. In the case of the Cr pig iron, however, it was quite unknown whether oxidation of P (dephosphorization) will satisfactorily occur or not, if the oxygen potential of the iron bath is lowered in order to control oxidation of Cr.
As for the denitrification of the Cr pig iron, two of us noticed that alkali metal carbonates have some effect for denitrifying as well as dephosphorizing the Cr pig iron, and patent applications were made therefor in Japan (Japanese Laying-Open Patent Publication No. 84113/77 and No. 023816/78). In these inventions, the molten Cr pig iron is contacted with neat alkali metal carbonates or a slag containing not less than 30% by weight of alkali metal carbonates, and as the slag ingredients SiO.sub.2, CaF.sub.2, Fe.sub.2 O.sub.3, CaO, etc. are referred to. These ingredients were intended for merely reducing vaporization loss of alkali metal carbonates. No definite idea was established with respect to slag composition, and it was considered that the principal role of denitrification and dephosphorization was played by the alkali metal compounds through and through.
After repeated experiments, we found that a slag comprising Li.sub.2 O or Li.sub.2 CO.sub.3 (the Li compound), a fluoride or chloride of an alkaline earth metal, and an oxide of Fe or Ni is effective for dephosphorization as well as denitrification and we provided a process for dephosphorization-denitrification of molten pig iron containing not less than 3% Cr, comprising maintaining the Si content of said molten iron at 0.2% by weight or less, contacting said pig iron with a slag comprising 30-80% by weight of at least one selected from fluorides and chlorides of alkaline earth metals, 0.4-30% by weight of at least one selected from lithium oxide and lithium carbonate (the Li compound), 5-50% by weight of at least one of iron oxides and nickel oxide, and 0-40% by weight of at least one selected from oxides and carbonates of alkaline earth metals, while controlling oxidation of Cr, which is the subject matter of our co-pending U.S. application Ser. No. 159,097 filed June 13, 1980.
In the course of our study, we gradually came to notice that dephosphorization and denitrification of the Cr pig iron is effected not only with the slag containing the Li compound but also with the slag containing other alkali metal compounds. We further proceeded with study on this theme and we now provide a novel process for dephosphorization-denitrification of the Cr pig iron.